The objective of the proposal is to establish a research program study the role of the thymus in T lymphocyte differentiation throughout the adult lifespan. The first goal of the program will be to develop a model to test the hypothesis that the thymic influence on T cell development takes place in two separate compartments: intrathymic and extrathymic. The model will use thymus epithelial cell (TE) cultures to simulate the intrathymic compartment. T6-chromosome marked bone marrow cells (T6-BM), some of which are prethymic precursors, will be treated with anti-Thy-l to remove contaminating T cells. The T6-BM will be cultured on TE to induce T cell differentiation, and after 104 days incubation, the T6 cells will be transplanted into neonatally thymectomized (NTx) syngenic hosts which lack the T6 chromosome marker. Thymus glands from 1-3 day old mice will be enclosed in cell-impermeable diffusion chambers and implanted into the appropriate NTx hosts to provide the thymic humoral factors of the extrathymic compartment. The model will be used to: 1) Determine if bone marrow cells cultured on TE give rise to immunocompetent T cells when transplanted into syngenic hosts, 2) Determine if bone marrow cells cultured without thymus humoral factors in the extrathymic compartment, 3) Determine if bone marrow cells cultured on TE are capable of restoring T cell functions to T cell deficient mice, 4) Determine if physical contact between precursor and inducer cells is essential for the differentiation of immunocompetent T cells. Verification of the model will allow it to be used to study the effect of advancing age on thymus influence on T cell homeostasis. Knowledge of the mechanisms by which the thymus influences changes in T cell homeostasis during an individual's lifespan may lead to methods for correcting deficiencies or alterations which would otherwise result in decreased immunological functions or pathological sequelae.